July 2011 Weather Recap

It was a cool 92 F in central Iowa on Sunday. Cool at least by July’s standards in which the state was blanketed by record high temperatures and unbearable heat indices. The month got off to a promising start with maximum daytime temperatures during the first half of the month primarily in the 80’s.

A brutal stretch of weather began on the 15th when the daytime temperature reached 93 F. Central Iowa wouldn’t see temperatures back in the 80’s for another 12 days. The record stretch of 13 consecutive days in the 90’s was the longest stretch in over 30 years. Not to be outdone, a 15-day stretch of nighttime temperatures at or above 74 F was the longest on record.

Overall, the daily average temperature for the month was 81.8 F which is 5.7 degrees warmer than the average. This ranks as the 4th warmest July on record for central Iowa. The warmest stretch occurred on the 17th through the 20th when daily temperatures nearly reached triple digits each day. Needless to say, these prolonged, extreme temperatures took their toll on our cool-season grasses.

Turf under severe environmental stress is particularly susceptible to mechanical damage.  Heat tracking occurred on this turf that was already experiencing heat and drought stress.

The ideal temperatures range for shoot growth of cool-season grasses is between 65 and 75 F and the optimum temperature range for root growth is somewhere between 50 and 65 F. The temperatures we experienced during the month of July have contributed to the summer decline of many of our turfgrasses. Summer decline is a general term that refers to a complex of stresses that leads to the decline of cool-season grasses during the summer months.

As an example of the environmental stress that our cool-season grasses have experienced recently, I used a device that measures canopy temperatures to see just how uncomfortable it has been for them. The picture below shows canopy temperatures from four different environments. These measurements were taken on Monday, July 18 in the middle of the afternoon.

Canopy temperatures of four different surfaces on a warm July afternoon.

As you can see, turf provides a dramatic cooling effect with canopy temperatures approximately 23 degrees cooler compared with a concrete surface and approximately 33 degrees cooler than organic mulch. Turf receiving shade under the canopy of a tree was about 15 degrees cooler compared with turf in full sun.

Warm soil temperatures are also contributing to summer decline. Roots can suffer dieback when soil temperatures are above 75 F for extended periods. Below is a list of 4-inch rootzone temperatures that influence certain biological and physiological events in the soil and in cool-season grasses.

90 F - Shoot growth comes to an end.
75-77 F - Root growth is compromised.
70 F – Root growth begins to slow.

The soil temperatures across much of the state are between 79 and 85 F. This means that root growth has stopped and dieback is probably occurring. The turf is under severe environmental stress and additional stresses from agronomic practices can further lead to summer decline. It may be necessary to re-evaluate and reduce some of your agronomic practices and only perform those that are absolutely necessary until the stress level declines.

Marcus Jones
Assistant Scientist